Davis takes
Often times on hikes that I con-
ducted for school children on St.
Croix, I explained how plants manu-
facture food. It Is good to know tha
plants are used as medicine, food
and other benefits from plants that
sustain mankind on earth. Those
who went on hikes with me over the
years have gained traditional and
scientific knowledge about them-
selves, their culture and other things
we discussed pertaining to our rela-
tionship with the environment.
Plants play a key role in the envi-
ronment by using the sunlight to
manufacture food for all living and
non-living things on earth. Green
plants, being autotrophic organisms,
can manufacture energy-rich organ-
ic substances by converting the radi-
ant energy they receive rom sun-
light into chemical energy present in
the bonds, which link the atoms of
organic molecules. This process is
called photosynthesis.
On hikes, I often asked students
why when it rains everything turns
green. What gives plants a green
color? How this or that plant pro-
duces food or looks or grows that
way. I believe that every child on a
hike with must learn something
about his/her environment In this
way. a child can walk awayknowing
something about the environment
and the role he or she plays in the
earth' ecosystems. "
Our earth receives from the sun a
constant stream of radiation in the
form of visible light whether it is
heat rays, radio waves, ultraviolet
light or x-rays. Of those lights from

a look at plants and photosynthesis

Olasee
Davis
Our
Environment

the sun, visible light and heat rays
profoundly affect the rates at which
plants can photosynthesize and
grow to produce food. Radiation
travels through space with a certain
wave motion. Although all the solar
radiation travels at the speed of light
at 186,000 miles per second, there is
considerable variation in the length
of the waves, that is, the number of
wave motions per unit distance.
You see, these light waves form
the visible spectrum, and it is light
in these wavelengths that plants use
in photosynthesis. However, the vis-
ible spectrum is only a small frac-
tion of the range of wavelengths
emitted by the sun. For example.
both shorter-wavelengths and longer
wavelengths reach the earth.
although we cannot see them direct-
ly. The amount of solar radiation
which reaches a plant on the earth's
surface will vary according to the
tranparency of the atmosphere, the
cloud cover. the position of the plant
on the earth, and the time of year.
Such native plant as the guavaber-
ry tree produces fruit late in the year
because the plant needs long nights
and short days. These are the kinds
of things students learned on my

hikes scientsflcally unit how the role
guavaberry plays traditionally dur-
ing our Christmas season. Many
physiological changes in plants are
related to the season of the year.
This Includes seed germination, bud
dormancy. flower and others. Short-
day plants flower when the day
length is shorter. Long-day plants
flower when the day is longer.
Whereas, day-neutral plants do not
depend on day length.
In 1771. Joseph Priestley per-
formed an experiment. which led to
the term photosynthesis. He placed
a mouse in a sealed jar. and
observed that it died after a while.
Then he put the mouse in a jar.
which a candle had burnt, and the
mouse died immediately. Priestly
found out that the candle had
exhausted the air. In his third exper-
iment, he put a twig of a plant in
with the exhausted air. after a while.
he added the mouse, and It did not
die immediately. "Vegetation. he
wrote. "restored the air. From these
discoveries we are assured that no
vegetable grows in vain but cleanses
and purifies our atmosphere."
Today, chemists can now explain
Priestley's experiment in terms of
the change of gas between organ-
isms and the atmosphere. The burn-
ing candle had exhausted the air of
oxygen, and without a supply of this
gas for respiration, the mouse died.
The plant twig used the carbon diox-
ide remaining in the air to restore
oxygen to the air by the process of
photosynthesis. The chemical equa-
tion of photosynthesis is Sunlight +

6C02 6H20-C6H I2*602, .
equation represents sunlight, ca.i
dioxide, water, sugar and oxygen
Photosynthesis occurs prim,i
in the leaves ofgreen plants. A ,.,
section of typical leaf will ri'.
that it is eminently suited to 1.
photosynthetic factory The Ic.;,
thin and flat to maximize the suri..,
area of the leaf thai receives -
light. It contains veins ofconduct.
tissue. so that water can he bros,1
in and the sugar formed can
transported to other pans on i
plant. The many air spaces or hole
showed students on leaf surface .
cells that facilitate gas exchange
In addition, the leaf surfalmi
especially the lower one, are d'ik
with thousands of tiny pores ciulk
stomates. These pores regulate ,*
exchange between the leaf and thi
atmosphere by their number at.
size. The plant can regulate the si,
of the stomates. For example. dutr
the day, when sunlight is availsah
the stomates open and carbon do,
ide enter the leaves. At night, wii,
there is no sunlight and photosyv
thesis does not occur, the stomati
are closed. The cells nearest th
upper surface of the leaf also s.,
tain many chloroplasts the ccth,
lar structures which are the site i
photosynthesis.
In next weeks column. I will c",
tinue talking about photosynthesis.
This article reflects the wet
Olasee Davis, a St. Croix ecology,
activist and writer who has a nma,'
of science degree in range mauay.
meant and forestry ecology